A paramagnetic oxygen sensor employs the following principles. An atom consists of a nucleus that is surrounded by orbiting electrons. An orbit can be occupied by up to two electrons, and one or more orbits make up an electron shell. In addition, each electron spins around its own axis and has a magnetic moment associated with the electron spin. The magnetic properties of the whole atom are then determined by the combined effect of the spins of all the electrons. Two paired electrons in the same orbit have opposite spins, which cancel their magnetic effects. However, oxygen is one of the rare molecules that has unpaired orbiting electrons around the nucleus and thus a magnetic property. It has an even number of electrons orbiting around the nucleus, but two of them are in unpaired orbits. This makes the oxygen molecule strongly susceptible to interaction with an external magnetic field.
The strength of the interaction between a molecule and a magnetic field is called magnetic susceptibility. Substances having positive magnetic susceptibility are called paramagnetic and those with negative susceptibility, diamagnetic. Positive susceptibility means that a molecule is attracted by a magnetic field, negative susceptibility means that it is repelled by it. Oxygen is the only gas that is paramagnetic, whereas other gases are weakly diamagnetic. This physical phenomenon offers a specific way to measure the oxygen content of a respiratory gas mixture, even when nitrous oxide is present.
In 1968 H. Hummel presented a way of using the paramagnetic principle by constructing a cell in which two gases were mixed inside a homogenous magnetic field. By using an alternating magnetic field, it is possible to measure a difference in pressure between the gases in two conduits upstream of the cell. The amplitude of this signal is directly proportional to the difference in oxygen partial pressure between the two gases to be measured. See U.S. Pat. No. 3,584,499. When the active volume of the measuring cell is made very small, the response time is fast enough for breath-by-breath measurements. The cell was commercialized, but it was bulky in size and sensitive to external vibrations and pressure.
The Datex Division of the Instrumentarium Corporation studied and further developed the Hummel cell configuration into a compact, fast, differential cell for measuring oxygen consumption. This oxygen analyzer is described in U.S. Pat. No. 4,633,705. The analyzer, which basic configuration is shown in FIG. 1, is constructed of an electromagnet with a thin air gap ensuring an essentially constant magnetic field between its poles. The gas to be measured and a reference gas are conducted to this air gap where they are mixed in the uniform magnetic field and then the mixture is conducted out from the gap. A reference gas is needed to measure the absolute oxygen fraction of the measured gas. A pressure difference proportional to the content of oxygen in the gas exists inside the three conduits entering the gap with a fixed magnetic field. If the oxygen content in the two gases differs, a pressure difference will exist between the inlet conduits outside the gap when the magnetic field is on. By selecting a proper switching frequency, the generated pressure signal can be detected by a differential pressure transducer connected between the inlet conduits. Although this oxygen analyzer is very compact, fast, and accurate it still has a few disadvantages.
The pressure signal is not measured in the exact spot where it is generated. The signal is transferred to the differential pressure sensor via tubing, connectors and additional volume, which moderate the signal amplitude. The associated pressure transfer function also depends on the properties of the gas and results in asymmetry between the two branches of the differential pressure sensor. Interfering mechanical background signals cause common-mode error, the magnitude of which is affected by the pressure sensor and gas composition. Some errors are introduced when the velocity of the gas changes, as occurs with changes either of the pump power or gas viscosity. The end of the reference tubing is usually under ambient pressure, but the sampling tubing is connected to a breathing circuit. The external pressure disturbance, the overpressure generated by the ventilator, is transmitted into cell and is detected by the pressure sensor. Another type of disadvantage is the continuous need of reference gas flow, which is a disadvantage in closed-circuit anesthesia when room air cannot be used as a reference, because it would result in a slow accumulation of nitrogen in the breathing circuit.